Radio receiver with loop antennas



Dec. 16, 1941.

v H. c. FORBES RADIO RECEIVER WITH LOOP ANTENNAS Filed Aug. 28, 1940 2 Sheets-Sheet l d mzq ATTORNEY 4 I Dec. 16, 1941. 2,266,630

H'. C. FORBES 11111310v RECEIVER WITH LOOP ANTENNAS Filed Aug. 26, 1940 2 Sheets-Sheet 2 I. "HI

SUPER BY 97 12: 400/0 32 Oar/44A 70k.

ATTORNEY Patented Dec. 16, 1941 RADIO RECEIVER WITH LOOP ANTENNAS Henry 0. Forbes, Bufialo, N. Y., assignor to Colonial Radio Corporation, Bufialo, N. Y.

Application August 28, 1940, Serial No. 354,539 iCl-aims. (o1.250 1i) This invention relates to radio receiving apparatus and more particularly to radio receiving apparatus equipped with loop antennas. It has its greatest utility in radio receivers of the socalled table model type intended particularly for broadcast listeners, but it will be understood that my invention is not limited thereto and is capable of use in other types of radio receivers and for other purposes.

It is known that reception may be had from loop antennas as well as from other types of antennas, and it is known that loop antennas have a directional characteristic, the pick-up being a maximum when the plane of the loop is in the line of the incoming signal and a minimum when it is at right angles thereto.

In recent years, it has become customary to equip broadcast receivers with built-in loops. In some cases, the loop has been fixedly mounted in the receiver, so that its direction could not be changed except by moving the receiver. In other cases, arrangements have been provided by means of which it was possible to rotate the loop through a limited amount within the receiver.

Neither of these arrangements has proved satisfactory for broadcast listeners as a whole. In the first case, that is, wherethe loop is fixed in position in the receiver, it is obviously impractical for the listener to move the entire receiver to be sure that in each case the loop is set in the proper direction for maximum signal, or for minimum interference.

In the second case, that is, where the loop is arranged for rotation in the receiver, in some instances it has been necessary to move the receiver in order to get at the loop to rotate it, and in other instances, mechanism has been provided having a control on the front or some other accessible part of the receiver by means of which the listener might rotate the loop. Such arrangements have tended to be unduly expensive because of the space requirements for the rotatable loop and because of the complexity of the mechanismrequired to rotate it; and even where such apparatus is provided it is not easy for the layman to find the particular angle of the loop for best results, and in any case such apparatus has not been possible for installation in so-called table model receivers where the space requirements are such that it is not possible to provide a rotatable loop.

Among the objects of my invention are to provide a loop installation which is easily assembled in table models without requiring any substantial amount of space and by means of which the lay operator may easily take advantage of the directional characteristics of the loop without moving the receiver itself and without the employment of complicated or expensive mechanisms.

A further object of my invention is to provide apparatus of the class described which may be readily installed in table model receivers and which may be operated by a simple push button .switch to vary the directional characteristic of the loop installation.

Still another object of my invention is to provide a loop installation which may be operated substantially without directional effects, if desired, or which may operate with directional effects if that is desired.

Still a further object of my invention is to provide a loop arrangement which may be readily installed in table models and operated easily and quickly and without complication by the layman either to give directional reception or to give substantially non-directional reception without requiring movement of the radio receiver.

Still other objects and advantages of my invention will be apparent from the specification.

In this application I have particularly pointed out and distinctly claimed the part, improvement or combination which I claim as my invention or discovery, and I have explained the principles thereof and the best mode in which I have contemplated applying those principles, so as to distinguish my invention from other inventions.

In the drawings:

Fig. 1 is a perspective view of a table model receiver according to my invention;

Fig. 2 is a circuit diagram of the antenna and input circuit which may be employed in accordance with my invention;

Fig. 3 is a polar diagram of the response of the apparatus of Fig. 2;

Fig. 4 is a circuit diagramof a modified form of my invention; and

Fig. 5 is a polar diagram of the response of the apparatus of Fig. 4; and.

Figs. 6, 7 and 8 are circuit diagrams of still further modified forms of apparatus according to my invention.

Referring now more particularly to Figs. 1 and 2, I have shown in Fig. 1 one form of table model receiver, and in this instance I may provide within the housing of the receiver the two loops l' and 2 mounted at right angles to each other and arranged in any convenient place within the cabinet. Both loops preferably have the same inductance and capacity, so that they may be interchanged in a tuned circuit without substantially affecting its tuning. For example, the loop I may be arranged at the back and extending along the back, whereas loop 2 may be at one end and extending along the end.

One terminal of the loops l and 2 may be connected together and through condenser 3 to ground and through resistance 4 to the automatic volume control line. The opposite terminal of loops I and 2 may be connected to switch points 5 and 6 adapted to be alternately engaged by the contact I, which may be connected through tuning condenser 9 to ground and to the control electrode I2b of tube I2 having cathode I2a, other grids I20 and I2d and an anode l2e.

The switching mechanism embodying contacts 5, 5 and I is preferably a push button type of switch which takes alternate position with successive pushes. For example, it may have the push button 8 extending from the front face of the housing and when first pushed in may connect contact 6 to grid I21) and such connection will preferably remain until further actuation of push button 8, at which further actuation contact I is disconnected from contact 6 and connected to contact 5.

As will be observed, when contact I is connected to contact 5, the antenna circuit is made up of loop I only, and condenser 9 is connected across the said loop to ground to which the opposite end of loop I is connected, thus forming a tuned input circuit for tube, I2. Tube I2 may be the input tube of a superheterodyne receiver and may be a pentode as shown, or other type of tube, the same being well-known to those skilled in the art. If desired, the receiver may be a tuned radio frequency amplifier type instead of a superheterodyne, my invention not being concerned with the type of circuit employed other than as herein described and claimed. In operation after the receiver has been turned on, as for instance by means of switch and volume control knob 34, the operator may tune in the desired station either by means of a manual knob III or by means of one of the push buttons I I and the signal may be heard from a loud speaker behind openings I3. After the desired station has been tuned in, the push button 8 may be pushed a few times to be sure that the loop is used which affords either the best reception or the minimum interference, as the case may be. The effect of pushing the button is to substitute the loop 2 for the loop I or the loop I for the loop 2 in the input circuit.

Referring now to Fig. 3, it will be noted that if the dotted line circles Ir represent the response of loop I, then the dotted line circles 21 represent the response of the loop 2, each response curve being in the form of a figure 8, and the two figure eights being disposed at 90 to each other. The over-all response which may be obtained is indicated by the outside full line curve. Actually this curve should be superimposed upon the dotted lines adjacent to it, but for purposes of clarity, it has been slightly separated therefrom. By inspection of Fig. 3, it will be noted that no matter from what direction the incoming signal may come, it is always possible to get a response which is between the response which would be obtained if the loop pointed directly in line with the signal and a minimum of 0.707 of that value, this because it is always possible by operating the switch 8 to select the loop which is no more than 45' out of line with the desired signal.

Referring now to Fig. 4, I have shown an arrangement in which the loops may be used together so as to obtain the pick-up of both loops. In this instance I have indicated another Variable condenser I4 which may be ganged with the variable condenser 9, the condenser I4 having one terminal connected to the high potential end of loop I and condenser 9 having one terminal connected to the high potential end of loop 2, the other terminals of both condensers being grounded. In this instance the common point of condenser I4 and loop I may be connected to switch point 5, the common point of condenser 9 and loop 2 to the switch point 6, and the switch blades I may have an intermediate position in which it makes contact with both terminals 5 and 6 connecting the tuned circuits in parallel.

Operation of the push button 8 may then connect the grid I2b in sequence first to switch point 5 only, then to switch points 5 and 6 and then to switch point 6 only. In the first position the input circuit is made up of loop I and condenser I4; in the third position it is made up of loop 2 and condenser 9; in the second position it is made up of both tuned circuits in parallel.

Since in switching from either of the single tuned circuits to both the tuned circuits in parallel the inductance is halved and the capacity nearly doubled, the circuits will remain nearly in tune to the same frequency when changing the switch position. Such minor differences in capacity, which may result from the minimum capacity of the tube and the switching circuit may be compensated for, so that no disturbance of tuning is caused in making the change.

The effect of changing the input circuit is to change the directional characteristic which is indicated in Fig. 5, the full line circles as before representing the characteristic of the loop I, the dotted circles representing the characteristics of the loop 2, and the dot and dash circle representing the characteristics of the loops as used in parallel or in series.

It will be noted by reference to Fig. 5 that the characteristic of both loops used together is directional as before and is the same figure eight characteristic. It will be noted, however, that it is shifted 45 from the axis of the characteristic of the loops I and 2 and is larger in magnitude. It is in fact the characteristic which would be obtained from a loop larger than either I or 2 and placed at 45 with respect to both of said loops. It may be observed that the directional axis of the characteristic of the two loops used together may be rotated by changing the polarity of either loop I or 2 with respect to the other, and if desired, additional contacts may be placed upon the switch 5, 6, I for this purpose.

Instead of using both tuned circuits in the input of the first tube I2, these circuits may be employed in the input of separate tubes, the output of which is combined in a subsequent tube as shown in Figs. 6, 7 and 8. In the circuit shown in Fig. 6 condenser I4 may be connected across loop I and condenser 9 across loop 2, the com mon point of these circuits being led to a midpoint between resistances 22 and 24 connected between the cathodes 20a and 2Ia of tubes 20 and 2 I, the output circuits of which are connected together and feed a common coupling circuit diagrammatically shown. The control electrode 201) of tube 20 may be connected to the common point of loop I and condenser I4 and the control electrode 2Ib to the common point of loop 2 and condenser 9.

Resistances 22 and 24 may be shunted by suitable by-pass condensers 23 and 25. The switch points 5, 6, and I may be connected as indicated, the switch point I to the mid-point between resistances 22 and 24, switch point 5 to cathode 20a and switch point 6 to cathode 2Ia. The biases upon these tubes which may be derived through or impressed across resistances 22 and 24 may be made such that when switch blade I is on switch point 5, tube 2I is biased to cut-off, whereas tube 20 operates with its normal bias, and similarly, when switch blade I is on switch point 6, tube is biased to cut-oil and tube 2| operates with its normal bias. Thus it will be seen that by turning the switch blade 1 to switch point 5, only the response of loop I will be obtained, whereas turning it to switch point B only the response of loop 2 will be obtained.

In case it is desired to provide for a smooth and gradual change of the characteristic of loop I to the characteristic of loop 2, the arrangement of Fig. 7 may be utilized, in which switch points 5, 6 and switch blade 5 have been replaced by resistors a and 30b, slider 3! having two arms 3m and til) making contact with resistors 30a and 3011 respectively. Slider 3! may be continuously varied from one end of resistors 36a and 30b to the other.

Cathode 20a may be connected to one end of resistor 36a, for instance the top end, the other end of which may be grounded, and cathode 21a may be connected to the lower end of resistor 3%, the upper end of which may be grounded. The biases are so chosen on tubes it and 2! that when the slider 3| is 'at the top end of resistor 30, tube 2! is biased to cut-01f, whereas tube 20 has its normal operating bias. By moving the slider St to the bottom extremity of resistor 30, tube 2| has its normal operating bias and tube 20 is biased to cut-off. For intermediate positions, an intermediate efiect is obtained which is similar to what would be obtained if loop I or 2 were rotated; that is to say, the directional pattern gradually and smoothly changes from that of loop 1 to that of loop 2 and vice versa.

It may now be seen that if it were possible to continuously operate the slider 3| from one extremity of its travel to the other and back again at a rate above audibility, such as 20,000 times per second, there would be no observable maximum or minimum directional characteristic in the loop. The efiect would be substantially the same as if the loops i or 2 were rotated at a rate above audibility. In other words, the system while having the convenience of loop reception, would have a directionless characteristic.

It is of course impractical to attempt to swing the slider 3! at such a rate, but a similar result may be obtained by varying the bias on tubes 20 and 21 at a super-audible rate as shown in Fig. 8. For example, across the terminals of the resistor 30, I may connect the secondary of transformer 33, the primary of which may be connected to the output of a superaudio frequency oscillator 32. This oscillator may operate at a superaudible frequency such as 20,000 cycles and the magnitude of its output is so chosen that the bias alternately impressed upon tubes 20 and 2! is sufficient to cut old one or the other. The efiect of this is that during a portion of each cycle the tube 20 only operates at its normal bias and during another portion of the oscillator cycle, the tube 2| only operates at its normal bias and the effect is as though the loops l and 2 were rotated at super-audible frequency, thereby eliminating the directional characteristic.

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made without departing from the spirit and scope of my invention as will be clear to those skilled in the art.

I claim:

1. A radio receiver comprising, in combination, a housing, a pair of vacuum tubes within said housing, each of said tubes having a cathode and a control electrode, said tubes having a common output circuit, a pair of loops mounted fixedly within said housing at angles to each other and having a common terminal, an impedance connected between said cathodes, a connection from said common terminal to said impedance, a connection from the free terminal of one of said loops to the control electrode of one of said tubes, a connection from the free terminal of the other of said loops to the control electrode of the other tube, selective means for applying a cut-off bias to either of said tubes and operating bias to the other of said tubes, said means comprising a manually operable two-position switch for connecting said common terminal to the cathode of either of said tubes.

2. A radio receiver comprising in combination a housing; a pair of vacuum tubes within said housing, each of said tubes having a cathode and a control electrode, said tubes having a common output circuit; a pair of loops mounted fixedly within said housing at angles to each other and having a common terminal; a resistance connected between said cathodes; a connection from said common terminal to an intermediate point on said resistance; a connection from the free terminal of one of said loops to the control electrode of one of said tubes; a connection from the free terminal of the other of said loops to the control electrode of the other tube; selective means for applying a cut-ofi bias to either of said tubes and an operating bias to the other of said tubes, said means comprising a two-position switch for connecting said common terminal to the cathode of either of said tubes.

3. A radio receiver comprising in combination a housing; a pair of vacuum tubes within said housing, each of said tubes having a cathode and a control electrode, said tubes having a common output circuit; a pair of loops mounted fixedly within said housing at angles to each other and having a common terminal; means for tuning each of said loops; an impedance connected between said cathodes; a connection from said common terminal to said impedance; a connection from the free terminal of one of said loops to the control electrode of one of said tubes; a connection from the free terminal of the other of said loops to the control electrode of the other tube; selective means for applying a cut-off bias to either of said tubes and an operating bias to the other of said tubes, said means comprising a two-position switch for connecting said common terminal to the cathode of either of said tubes.

4. A radio receiver comprising in combination a housing; a pair of vacuum tubes within said housing, each of said tubes having a cathode and a control electrode, said tubes having a common output circuit; a pair of loops mounted fixedly within said housing at angles to each other, and having a common terminal; means for tuning each of said loops alike and in unison; an impedance connected between said cathodes; a connection from said common terminal to said impedance; a connection from the free terminal of one of said loops to the control electrode of one of said tubes; a connection from the free terminal of the other of said loops to the control electrode of the other tube; selective means for applying a cut-off bias to either of said tubes and an operating bias to the other of said tubes, said means comprising a two-position switch for connecting said common terminal to the cathode of either of said tubes.

HENRY C. FORBEfi, 

